The present invention relates to the field of non-contact surface metrology. During many manufacturing operations it is important to measure the surface profile of a work piece. Over time, the accuracy and spatial resolution requirements of the surface profile have increased while at the same time the characteristics of the surface to be profiled have become more challenging to measure using conventional profiling technologies.
Historically, surface profiles were coarsely sampled with accuracies of 0.1 to 1.0 thousandths of an inch using dial indicators. Later, and to the present day, co-ordinate measuring machines (CMMs) replaced dial indicators but CMMs remain, essentially, highly accurate, point probe devices. That is, measurements are defined by scanning over the x-y surface with a probe attached to a third moving axis of the machine. Probes may be mechanical, optical, laser, or white light, among others.
A number of image-based metrology systems have been developed to overcome the inherent slowness of CMMs when used to measure large surface areas with high spatial sampling density. Perhaps the first image-based metrology technology was photogrammetry which was used for mapping purposes reportedly as early as the 1850's. For the purposes of this specification photogrammetry is the measurement of a surface profile by triangulation of a point's three-dimensional coordinates from two (or more) non-redundant images of the surface. Photogrammetric metrology systems have become popular as digital cameras and inexpensive computers have become commonplace. One difficulty encountered in using a photogrammetry system is identifying the pairs of image points (in the two images) on which to triangulate. Typically in industrial applications, where often the surface is relatively featureless, easily identifiable targets are placed on the work surface and triangulation is applied to these sparse targets.
A second image-based metrology system is based on “structured light” projection (SLP). SLP systems are a subset of photogrammetry that were developed to overcome the difficulty of identifying pairs of measurement points in both photogrammetry images. In effect, the “structured light” that is projected onto the work surface replaces the physical targets in more traditional photogrammetry systems.
Many different light patterns can be used in SLP systems. A pattern of small light spots is a simple example. If the spots are sufficiently spread out, or are uniquely shaped, then the system can uniquely identify a given spot in each of the two images. With the same spot identified in each image, triangulation can be accurately performed.
More typically in modern SLP systems the light pattern comprises one-dimensional sinusoidal fringes or binary stripes. Using a well-known phase shifting technology the SLP system assigns a sine phase to each point in each image—the phase for a given point on the surface is the same in each image. The SLP system then triangulates phase-matched points in each image.
Neither traditional photogrammetry systems nor SLP systems can operate successfully on certain classes of work surfaces. For example, many lightweight structures are fabricated as a laminated sandwich comprising smooth outer skins enclosing a cellular core. For high performance applications the face of the cellular core is machined to tight tolerances but neither existing technologies, as currently implemented, can measure the “virtual” surface formed by the ends of the machined walls of the cells with adequate accuracy or spatial resolution.